The invention relates to optical switching and free-space coupling of fiber optic waveguides in a single-mode fiber-optic transmission system, and specifically to calibrating the model terms of such systems. The invention finds application to micro-electromechanical systems (MEMS), but it is not so limited.
The technique of using open-loop oscillatory mirror commands (dithers) may be used in optical switching for: (1) detection of alignment errors through synchronous detection such as is described in a patent application of the present inventor entitled METHOD AND APPARATUS FOR OPTICAL BEAM ALIGNMENT DETECTION AND CONTROL (TTC Docket 20974-1.00), and for (2) intentional optical power attenuation through induced constant-power coupling loss such as is described in a patent application of the present inventor entitled METHOD AND APPARATUS FOR OPTICAL BEAM POWER ATTENUATION (TTC Docket 20974-8.00). In both examples, performance is limited by errors in the kinematics models that relate actuation commands to coupling loss. In the case of synchronous detection, modeling errors may create a cross-coupled closed-loop response that would necessitate reducing the servo bandwidths to achieve stable response. A A reduced bandwidth translates directly to an increased time required to nullify the alignment errors detected after the open-loop portion of a switch, which increases the overall switch time. In both of the detection and attenuation examples, modeling errors can cause undesirable signal content in the coupled power.
The kinematical relationship between mirror actuation commands and coupling loss is a strong function of the optical design (nominal path lengths, focal length, wavelength, and fiber properties) as well as unavoidable small errors in the component prescriptions and in the time-varying and temperature-dependent alignment of the optical components. The calibration values determined during an initial off-line calibration might provide acceptable performance for only a relatively short time after the calibration, after which the system might violate one or more performance specifications. If the system is required to operate longer than the time period of validity of the initial calibration, then the approach of using an initial off-line calibration is not acceptable. Taking a connection off-line for the purpose of calibration or injecting calibration-specific signals into the system may also be unacceptable options.
What is needed is a non-interference calibration technique designed to continuously detect and tune the parameters of a kinematical model used as part of a control system for a fiber-optic switch or similar optical system. This non-interference calibration technique must not require that any calibration-specific signals be injected into the system, or that the operation of primary functions such as alignment control and/or power attenuation be halted for the purpose of calibration.
According to the invention, a method is provided for calibrating a free-space-coupled fiber-optic transmission system. The method uses signal content in the measured coupled power at frequencies equal to the sum, difference, and first harmonics of the frequencies of control-related dither signals to infer and correct errors in the kinematical model used to apply the dithers. The technique can operate independently or simultaneously with dither-based functions such as alignment control and power attenuation to provide continuous tracking of model terms. The calibration method does not require that any calibration-specific signals be injected into the system, or that the operation of primary functions be halted for the purpose of calibration.
The invention is explained with respect to a structure with two beam-steering elements having four degrees of freedom. The invention is readily generalized to a single beam-steering element or to a larger number of beam-steering elements.
The invention will be better understood by reference to the following detailed description in connection with the accompanying embodiments.